Add support for specifying SDK version constraints in pubspecs.

utils/pub/sdk_source.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library sdk_source;
 
 import 'dart:async';
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'sdk.dart' as sdk;
 import 'source.dart';
 import 'version.dart';
 
 /// A package source that uses libraries from the Dart SDK.
 class SdkSource extends Source {
   final String name = "sdk";
   final bool shouldCache = false;
 
   /// SDK packages are not individually versioned. Instead, their version is
   /// inferred from the revision number of the SDK itself.
   Future<Pubspec> describe(PackageId id) {
     return _getPackagePath(id).then((packageDir) {
       // TODO(rnystrom): What if packageDir is null?
       return Package.load(id.name, packageDir, systemCache.sources);
     }).then((package) {
       // Ignore the pubspec's version, and use the SDK's.
-      return new Pubspec(id.name, sdk.version, package.pubspec.dependencies);
+      return new Pubspec(id.name, sdk.version, package.pubspec.dependencies,
+          package.pubspec.environment);
     });
   }
 
   /// Since all the SDK files are already available locally, installation just
   /// involves symlinking the SDK library into the packages directory.
   Future<bool> install(PackageId id, String destPath) {
     return _getPackagePath(id).then((path) {
       if (path == null) return new Future<bool>.immediate(false);
 
       return createPackageSymlink(id.name, path, destPath).then(
           (_) => true);
     });
   }
 
   /// Gets the path in the SDK's "pkg" directory to the directory containing
   /// package [id]. Returns `null` if the package could not be found.
   Future<String> _getPackagePath(PackageId id) {
     var pkgPath = join(sdk.rootDirectory, "pkg", id.description);
     return dirExists(pkgPath).then((found) => found ? pkgPath : null);
   }
 }